Today hydrogen is considered to be a promising energy resource. Unfortunately, its application runs up against production difficulties. In fact, the yields for industrial hydrogen production, in particular starting from water less polluting than production from hydrocarbons, are still unsatisfactory.
Nonetheless, some methods are encouraging, such as High-Temperature Electrolysis (HTE). HTE consists of electrolyzing steam at a temperature from 500° C. to 1000° C. using electric energy supplied by means of a high temperature electrolyzer. Such an electrolyzer includes a plurality of cells. Each cell includes a porous cathode and a porous anode and also an electrolyte impermeable to the gases. The electrolyte is arranged in contact with the cathode and the anode and in between them. A voltage is applied between the cathode and the anode. A flux of steam is directed towards the cathode. The voltage applied causes the cracking of the water: the water is transformed into hydrogen and oxygen. The hydrogen is released from the cathode side while the oxygen is released from the anode side:
reduction at the cathode: 2H++2e−→H2;
oxidation at the anode: 2H2O→O2+4H++4e−;
global reaction: 2H2O→2H2+O2.
The overall energy necessary to allow this reaction can be supplied by electric energy coming from an electrical source such as an electric power plant and/or by thermal energy coming from a heat source.
The electrolytic reaction by itself is an endothermic reaction. Thus, if the electric energy supply is insufficient to cover the needs both electrical and thermal (via resistive heating taking place inside the one or more stacks of cells), then the electrolytic reaction is going to drain thermal energy from the gas traversing the cells which would end with a drop in the temperature inside the enclosure in which the cells are arranged: this is the endothermic mode of operation.
Solutions exist for operating an electrolyzer at high temperature in endothermic mode. For example, document WO2013/060869 describes a thermal management process for a high temperature electrolyzer, wherein the energy is stored by means of energy storage during a phase of exothermic operation of the electrolyzer (meaning that the electric energy supply is greater than the needs of the electrolytic reaction), for example at 1.5 V, when the electric energy price is low and wherein this stored energy is recovered in the form of heat during an endothermic operating phase of the electrolyzer, for example 1.2 V, when the price of electric energy is high.
This solution is satisfactory when the electrolyzer is connected to an electric source which produces sufficient energy. The exothermic mode/endothermic mode cycle, meaning the length of the exothermic mode and the length of the endothermic mode, is determined by the operator who can consequently choose the storage means suited to this cycle.
The HTE in combination with a renewable electric energy source, such as wind, water current, solar, hydroelectric, geothermal etc. could allow the production of hydrogen from renewable energy sources. Unfortunately, renewable electric energy sources are generally intermittent and/or fluctuating sources which do not allow continuous, constant and predictable energy production.
Consequently, the process from document WO2013/060869 can only be partially used in such a case.
Additionally, the electrolyzer includes elements which are sensitive to thermal gradients, meaning to a temperature difference between 2 points or 2 stacks of cells. An excessive temperature variation between different points of the electrolyzer can deteriorate these elements or even lead to their destruction.
Thus, in the case where the electrolyzer is supplied by renewable electric energy source, it is possible that the energy supplied be extremely insufficient. The electrolyzer then drains the missing energy from the environment where it is located, meaning from the heat stored in the enclosure, leading to the appearance of thermal gradients within the enclosure. These thermal gradients can then go beyond 50° C./cm.
The patent FR2,921,390 provides a solution for managing the thermal gradients in the one or more stacks of cells constituting the high temperature electrolyzers. The selected solution consists of circulating steam inside the one or more stacks of cells via independent conduits in order to homogenize the temperature before directing it towards the cathodes. In exothermic mode, the excess heat is therefore evacuated via a circulation of steam having a temperature below the temperature of the cells. In endothermic mode, the heat deficit is supplied via circulation of steam having a temperature greater than the temperature of the cells. This solution can only be implemented in endothermic mode if a source of very high temperature steam is available.
The disadvantage of this solution is that it requires providing a fluid circuit in order to conduct the steam through the various cells of the electrolyzer. Thus, this solution is architecturally complex. Additionally, when the power supplied by the energy source is very insufficient (endothermic mode), a very high temperature heat source must be available to cover the thermal needs of the reaction.
Thus the solutions from the prior art still require improvements in order to be able to propose a process with which to operate an electrolyzer at high temperature supplied with electricity by an intermittent energy source and in particular a renewable energy source.